The invention relates to a high-pressure sodium discharge lamp comprising a discharge vessel which is enclosed with intervening space by an outer bulb and has a ceramic wall, in which two electrodes with respective tips interspaced by a distance D are present, the discharge vessel having a substantially circular cross-section with an internal diameter d.sub.i at least over the distance D, while the outer bulb contains a gas filling, which lamp radiates white light with a colour temperature T.sub.c of at least 2500 K. under nominal operating conditions.
A lamp of the type described in the opening paragraph is known from DE-OS No. 31 29 329. The known lamp radiates white light under operating conditions and has a relatively high luminous efficacy. The colour rendering of the light radiated by the lamp expressed as the general colour rendering index R.sub.a is above 80 under certain conditions. In case of the colour rendering index R.sub.a &lt;80, the lamp can serve as a replacement for incandescent lamps. For this, however, it is desirable that the colour temperature T.sub.c should be considerably higher than 2500 K., since the colour temperature of incandescent lamps is between 2600 K. and 4000 K. Generally, light radiated by high-pressure sodium lamps can be regarded as "white" light if it falls within the region in the colour triangle bounded by straight lines through points having coordinates (X, Y), (0,400; 0,430), (0,510; 0,430), (0,485; 0,390) and (0,400; 0,360). The colour temperature in this case lies between approximately 2300 K. and 4000 K. By way of comparison it should be noted that the light of a standard high-pressure sodium discharge lamp, which radiates a golden-yellow light, has a T.sub.c &lt;2200 K. and an R.sub.a &lt;50. The luminous efficacy of this lamp, however, is considerably higher than that of the known lamp of the same power rating.
The known lamp has a high power rating, i.e. approximately 400 W or more, and thus has a relatively high luminous flux. The lamp can therefore only be used for large-scale illumination such as, for example, public lighting. A high-pressure sodium discharge lamp radiating light with very good colour characteristics (T.sub.c &gt;2500K., R.sub.a &gt;80) and so suitable as a replacement for incandescent lamps would seem to be highly suitable for interior lighting applications such as, for example, accent lighting. This requires a lamp of relatively small dimensions. Light with very good colour characteristics is also required for application in, for example, a motorcar headlamp. Here, too, relatively small dimensions of the lamp are desirable. Lamps of a relatively low luminous flux and relatively small dimensions are wanted for such applications. There is thus a demand for lamps having a relatively low power rating and relatively small dimensions for applications of the known lamp. A colour temperature of at least 2800 K. with the highest possible colour rendering value is required for this in a great number of cases. A reduction of the rated lamp power to below 400 W, however, leads to a considerable drop of the colour temperature to below 2200 K. and of the colour rendering index to far below 50 in the known lamp. In fact, the known lamp is then a standard high-pressure sodium discharge lamp, which does not generate "white" light.
An article in the Journal of IES, Summer 1988, pp. 105-117 describes a high-pressure sodium discharge lamp of relatively low power rating which under nominal conditions radiates white light with a T.sub.c of 2500 K. and an R.sub.a of about 80. The luminous efficacy is just under 40 lm/W. An increase in the colour temperature, whereby R.sub.a remains at least 80, is only possible through an overload, that is to say by increasing the power supplied to the lamp to above the rated power. However, this is accompanied by a sharp drop of the luminous efficacy on the one hand and a steep rise of the wall temperature of the discharge vessel to an undesirable level on the other hand. Under the overload conditions, a maximum R.sub.a is achieved at a colour temperature of 2700 K. The colour rendering deteriorates again when the colour temperature is further raised.